1. Field of the Invention
The present invention relates to a pattern formation method and a method and apparatus for production of a semiconductor device using that method, more particularly relates to a method of exposure which enables a pattern to be formed without the problem of a secondary peak even if using a phase shifting mask.
2. Description of the Related Art
At present, in the research and development of semiconductor integrated circuits, effort is being made to develop devices of a design rule of the sub-half micron order. In developing such devices, photolithography is indispensable. It is not too much to say that the resolution performance of the exposure devices used in photolithography, the so-called xe2x80x9creduction, projection, and exposure devicesxe2x80x9d, determines the success or failure of research and development into semiconductor devices and the feasibility of mass production.
Conventionally, the resolution performance of reduced projection and exposure devices has been improved by enlarging the numerical aperture of the reduction projection lens or shortening the exposure wavelength based on the following Rayleigh criterion:
R=k1xc3x97xcex/NA
where, R: resolution
xcex: Exposure wavelength
NA: Numerical aperture
k1: Process coefficient
However, in the fabrication of a semiconductor device, there are step differences caused by the topography, wafer flatness, etc. of the semiconductor device, and therefore securing of the depth of focus is also an important parameter at the same time as the resolution performance. The dimensional precision of the resist pattern in the photolithography step at the time of fabrication of a semiconductor device is generally xc2x15 percent. In an actual device, as shown in FIG. 1, there is always unevenness in the surface of the semiconductor substrate S. For example, there is a convex portion In of polycrystalline silicon etc. As a result, the pattern of the resist PR is not formed on the same focal plane. For this reason, the dimensions of the pattern of the resist PR differ between the upper portion and the lower portion of a step difference. Of course, this becomes more conspicuous the finer the pattern in a case where a stepper of the same wavelength and same numerical aperture is used. This tendency is seen in common for all types of resist.
The depth of focus becomes smaller in primary proportion to the exposure wavelength and in inverse proportion to the square of the numerical aperture. At the mass production stage, a depth of focus of about 1.5 xcexcm is necessary. For this reason, there are restrictions in order to satisfy both of the resolution performance and the depth of focus considered necessary.
FIGS. 2A and 2B show the dependency on the numerical aperture when the resolution performance of the depth of focus (D.O.F) in KrF excimer laser lithography, which is the most advanced exposure, is used as a parameter. As will be understood from the figures, the highest resolution which is obtained while satisfying the needed depth of focus of 1.5 xcexcm is about 0.35 xcexcm. Accordingly, it is extremely difficult to resolve a line width of 0.35 xcexcm or less with a depth of focus of 1.5 xcexcm or more. Some sort of technique for enlarging the depth of focus is necessary.
In response to such a request, in recent years, the halftone type phase shift method has been proposed. This exposure method is an extremely powerful method for improving the resolution and depth of focus of an isolated pattern such as a contact hole. In the halftone type phase shift method, as shown in FIG. 3, a semitransparent Cr, Six Ny, SiOx, Ny, Mox, Siy film, or the like having a transmittance with respect to the exposure light of about several percent to about 20 percent, that is, allowing passage of a fine amount of exposure light therethrough, is used as the halftone film 2 corresponding to the dark portions 1. In the bright portions 3, both the film 2 and the transparent substrate (with a concave portion 5 formed therein) or only the film 2 is etched and made to act as a mask. At this time, by setting the phase difference between the bright portions 3 and the dark portions 1 formed by the semitransparent film to 180xc2x0, as shown in FIG. 4B, the gradient of the distribution of the intensity of the light in an isolated pattern (for example, a hole pattern of 0.6 xcex/NA) can be made sharp. Note that, FIG. 4A shows the distribution of the intensity of the light in an isolated pattern using a conventional chromium mask.
In the design of this phase shifting mask, the transmittance of the halftone film 2 is an important factor. Namely, so as to make the gradient of the distribution of the intensity of the light in an isolated pattern sharper, it is sufficient to raise the transmittance of the halftone film 2. However, by raising the transmittance, the light shielding effect by the halftone film 2 is weakened, and the resist ends up exposed over its entire surface.
Also, usually, at the time of formation of a pattern, as shown in FIG. 5A, a secondary peak called a side lobe is generated due to the adjacency effect on both sides of the desired pattern position in the distribution of the intensity of the light irrespective of the light shielding position. The secondary peak becomes stronger by raising the halftone transmittance. As shown in FIG. 5B, even in a so-called completely isolated contact hole wherein, for example, when the design dimension of the hole pattern 6 is defined as W, the distance between the adjoining patterns is 3W or more, the peripheral portion becomes xe2x80x9cgougedxe2x80x9d in shape (numeral 8 part). With a shape as shown in FIG. 5B, there is concern that the diameter of the contact hole will be enlarged in the etching step.
Further, when it is intended to apply the halftone phase shifting mask method to a so-called periodic pattern portion having a high pattern density, the secondary peak becomes stronger due to interference between adjoining patterns, that is, the mutual adjacency effect, at the periodic pattern portion having the high pattern density.
Accordingly, when it is intended to form a device pattern by using the halftone phase shifting mask method, the design and a CAD process must be carried out with sufficient consideration given to the distance between patterns. This places a tremendous load on the design and CAD process and prevents practical application.
So as to solve the above-mentioned problems, intensive study is currently underway in various areas on how to enlarge the depth of focus without making the adjacency effect more conspicuous and without placing a heavy load on the design of the mask. However, no effective exposure method for enlarging the depth of focus without suffering from the above-described problems has yet been found. Accordingly, it is essential to quickly establish an exposure technique for enlarging the depth of focus without making the adjacency effect conspicuous and without placing a heavy load on the design of the mask.
At the present time, when a practical depth of focus cannot be obtained in the fabrication of a device, pattern formation has been carried out by using the multilayer resist method, electron beam exposure method, and the like. However, a sufficiently satisfactory effect has not been obtained.
Accordingly, it is essential to quickly establish an exposure technique for enlarging the depth of focus without making the adjacency effect conspicuous, without placing a heavy load on the design of the mask, and without exerting an advance influence on the aberration and other imaging characteristics by a method other than that mentioned above.
The present invention was made in consideration of the above-described situation and has as an object thereof to provide a pattern formation method and a method and apparatus for production of a semiconductor device using the same which, when fabricating a semiconductor device or the like, decides on the method of enlarging the depth of focus so that a stable resist pattern can be formed well even if the mask pattern thereof is fine and thereby enables a good resist patterning.
The present invention provides a method for enlarging the depth of focus at the time of fabrication of a semiconductor device without exerting an adverse influence upon the aberration and imaging characteristic by using an ordinary mask having any pattern density, in particular, a halftone phase shifting mask, even if the mask pattern of the semiconductor device is fine. The above-described object is achieved by this. This will be understood by analyzing the optical system of the exposure device. Namely, the present invention was obtained by the following discovery by the present inventor.
The depth of focus was enlarged without exerting an adverse influence upon the aberration and other imaging characteristics by using a halftone phase shifting mask having any pattern density by using the following means:
(1) So as to enable the use of the halftone phase shifting mask method for a design pattern having any pattern density without placing a tremendous load on the design and CAD process, it is sufficient if use may be made of the so-called periodic pattern portion having a high pattern density. Such a periodic pattern portion having a high pattern density is formed by interference of adjoining patterns, so it is sufficient if the secondary peak in the distribution of the intensity of the light due to the mutual adjacency effect can be reduced.
(2) The distribution of the intensity of the light depends upon the distribution of the intensity of the light in the effective light source of the exposure device. Namely, the optical projection system of the exposure device, that is, the xe2x80x9cstepperxe2x80x9d, as shown in FIG. 6, has a whole system diaphragm 10 arranged at first group and second group of focal planes. It is a dual telecentric imaging system and is an afocal system as a whole. Namely, the mask surface 12 and pupil surface (substantially equal to the diaphragm 10) and the pupil surface and wafer surface 14 are in a Fourier transformation relationship. Also, the light from the illumination optical system for illuminating the mask is shown developed completely as plane waves. Due to the afocal characteristic, individual plane waves correspond to individual light emission points of the secondary light source (also called the effective light source) 16, that is, the fly""s-eye lens.
(3) Based to the characteristic shown by (2), if the respective coordinate systems of the mask surface, pupil surface, and the wafer surface are defined as (x, y), ("xgr", xcex7), and (xcex1, xcex2) and the focal distances of the first group and second group are defined as f1 and f2 and, also, the transmission function of the mask is defined as o (x, y), the amplitude distribution f ("xgr", xcex7, xcex81, xcex82) on the pupil surface can be represented by the following equation:
f("xgr",xcex7,xcex81,xcex82)=∫∫o(x,y)exp{jk(sin xcex81x+sin xcex82y)}
exp{jk("xgr"x+xcex7y)/f1}dxdy=∫∫o(x,y)exp[jk{(sin xcex81+"xgr"/f1)x+(sin xcex82+xcex7/f1)y}]dxdyxe2x80x83xe2x80x83(1)
where, xcex81, and xcex82 are made the incident angles of the plane wave incident upon the mask in the x and y directions.
When defining the amplitude distribution on the wafer surface as g (xcex1, xcex2, xcex81, xcex82) and defining the pupil function as p ("xgr", xcex7), the following equation stands:
xe2x80x83g(xcex1,xcex2,xcex81,xcex82)=∫∫f("xgr",xcex7,xcex81,xcex82)exp{xe2x88x92jk(xcex1"xgr"+xcex2xcex7)}d"xgr"dxcex7=∫∫∫∫o(x,y)p("xgr",xcex7)
exp[jk{(sin xcex81+"xgr"/f1)x+(sin xcex82+xcex7/f2)y}]
exp{xe2x88x92jk(xcex1"xgr"+xcex2xcex7)}dxdyd"xgr"dxcex7xe2x80x83xe2x80x83(2)
The distribution of intensity I (xcex1, xcex2, xcex81, xcex82) of the plane wave incident on the wafer surface by xcex81 and xcex82 can be represented by the following equation:
I(xcex1,xcex2,xcex81,xcex82)=g(xcex1,xcex2,xcex81,xcex82)g*(xcex1,xcex2,xcex81,xcex82)xe2x80x83xe2x80x83(3)
Therefore, the overall distribution of intensity I (xcex1, xcex2) becomes the superposition of the plane waves, and therefore if the weighing function is defined as W (xcex81, xcex82), the following equation stands:
I(xcex1,xcex2)=∫∫w(xcex81,xcex82)I(xcex1,xcex2,xcex81,xcex82)dxcex81dxcex82xe2x80x83xe2x80x83(4)
Were, when assuming xe2x80x9cthe mask is a lattice pattern of equal intervals parallel to the y-axisxe2x80x9d, the transmission function o (x, y) of the mask can be represented by the following equation defining the mask frequency as xcfx89:                               o          ⁡                      (                          x              ,              y                        )                          =                              o            ⁡                          (              x              )                                =                                                                      ∑                                      n                    =                                          -                      ∞                                                        ∞                                ⁢                                                      a                    n                                    ⁢                                      exp                    ⁡                                          (                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        n                        ⁢                                                  xe2x80x83                                                ⁢                        ω                        ⁢                                                  xe2x80x83                                                ⁢                        x                                            )                                                                                  ⁢                              
                            -              ∞                         less than             n             less than                           +              ∞                                                          (        5        )            
Accordingly, the amplitude distribution f ("xgr", xcex7, xcex81, xcex82) on the pupil surface can be represented by the following equation:                                                                                           £                  ⁡                                      (                                          ξ                      ,                      η                      ,                                              θ                        1                                            ,                                              θ                        2                                                              )                                                  =                                  ∫                                      ∫                                          ∑                                                                        a                          n                                                ⁢                                                  exp                          ⁡                                                      (                                                          j                              ⁢                                                              xe2x80x83                                                            ⁢                              n                              ⁢                                                              xe2x80x83                                                            ⁢                              ω                              ⁢                                                              xe2x80x83                                                            ⁢                              x                                                        )                                                                                                                                                          ⁢                              
                            ⁢                              exp                ⁢                                  ⌊                                                            j                      ⁢                                              xe2x80x83                                            ⁢                      k                      ⁢                                              {                                                                              sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          θ                              1                                                                                +                                                      ξ                            /                            f1                                                                          )                                            ⁢                      x                                        +                                                                  (                                                                              sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          θ                              2                                                                                +                                                      η                            /                            f1                                                                          )                                            ⁢                      y                                                        }                                                      ⌋                    ⁢                      ⅆ            x                    ⁢                      ⅆ            y                          =                              ∑            n                    ⁢                                    a              n                        ⁢                          δ              ⁡                              (                                                      sin                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      1                                                        +                                      n                    ⁢                                          xe2x80x83                                        ⁢                                          ω                      /                      k                                                        +                                      ξ                    /                    f1                                                  )                                      ⁢                          δ              ⁡                              (                                                      sin                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      2                                                        +                                      η                    /                    f1                                                  )                                                                        (        6        )            
When the wave plane aberration on the pupil surface is W ("xgr", xcex7), the amplitude distribution g (xcex1, xcex2, xcex81, xcex82) on the wafer surface can be represented by the following equation:                                           g            ⁡                          (                              α                ,                β                ,                                  θ                  1                                ,                                  θ                  2                                            )                                =                                    ∑              n                        ⁢                                          a                n                            ⁢                              ∫                                  ∫                                                            δ                      ⁡                                              (                                                                              sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          θ                              1                                                                                +                                                      n                            ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              /                              k                                                                                +                          ξ                          +                          f1                                                )                                                              ⁢                                          δ                      ⁡                                              (                                                                              sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          θ                              2                                                                                +                                                      η                            /                            f1                                                                          )                                                                                                                                ⁢                  
                ⁢                                            p              n                        ⁡                          (                                                θ                  1                                ,                                  θ                  2                                            )                                ⁢          exp          ⁢                      {                          j              ⁢                              xe2x80x83                            ⁢                              W                ⁡                                  (                                      ξ                    ,                    η                                    )                                                      }                    ⁢          exp          ⁢                      {                                          -                j                            ⁢                              xe2x80x83                            ⁢                              k                ⁡                                  (                                      ξα                    +                                          η                      ⁢                                              xe2x80x83                                            ⁢                      β                                                        )                                                      }                    ⁢                      ⅆ            ξ                    ⁢                      ⅆ            η                          =                              ∑            n                    ⁢                                    a              n                        ⁢                                          p                n                            ⁡                              (                                                      θ                    1                                    ,                                      θ                    2                                                  )                                      ⁢                          exp              [                              j                ⁢                                  xe2x80x83                                ⁢                W                ⁢                                  {                                                            -                                              f1                        ⁡                                                  (                                                                                    sin                              ⁢                                                              xe2x80x83                                                            ⁢                                                              θ                                1                                                                                      +                                                          n                              ⁢                                                              xe2x80x83                                                            ⁢                                                              ω                                /                                k                                                                                                              )                                                                                      ,                                                                  -                        f1                                            ⁢                                              xe2x80x83                                            ⁢                      sin                      ⁢                                              xe2x80x83                                            ⁢                                              θ                        2                                                                              }                                            )                        ⁢                          exp              ⁡                              [                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      k                    ⁡                                          (                                                                        sin                          ⁢                                                      xe2x80x83                                                    ⁢                                                      θ                            1                                                                          +                                                  n                          ⁢                                                      xe2x80x83                                                    ⁢                                                      ω                            /                            k                                                                                              )                                                        ⁢                                      (                                          f1                      /                      f2                                        )                                    ⁢                  α                                ]                                      ⁢                          exp              ⁡                              [                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      k                    ⁡                                          (                                              f1                        /                        f2                                            )                                                        ⁢                  β                  ⁢                                      xe2x80x83                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      θ                    2                                                  ]                                                                        (        7        )            
Here, as apparent from the aforesaid equation (7), the spectrum distribution of the object on the pupil surface is the sum of xcex4-functions. Accordingly, a decision whether or not the n-th deffracted light enters into the pupil is clear. Therefore, use is made of a vignetting factor Pn for deciding whether the deffracted light should or should not enter into the pupil in equation (8).
Pn(xcex81, xcex82)=1 spectrum exit in pupil plane 0 otherwisexe2x80x83xe2x80x83(8)
From the above, the distribution of intensity I (xcex1xcex2, xcex81, xcex82) of the individual plane waves on the wafer surface can be represented by the following equation:                               I          ⁡                      (                          α              ,              β              ,                              θ                1                            ,                              θ                2                                      )                          =                              ∑            n                    ⁢                                    ∑              m                        ⁢                                          a                n                            ⁢                              a                m                            ⁢                                                p                  n                                ⁡                                  (                                                            θ                      1                                        ,                                          θ                      2                                                        )                                            ⁢                                                p                  m                                ⁡                                  (                                                            θ                      1                                        ,                                          θ                      2                                                        )                                            ⁢                              exp                [                                                      j                    ⁢                                          xe2x80x83                                        ⁢                    W                    ⁢                                          {                                                                        -                                                      f1                            ⁡                                                          (                                                                                                sin                                  ⁢                                                                      xe2x80x83                                                                    ⁢                                                                      θ                                    1                                                                                                  +                                                                  n                                  ⁢                                                                      xe2x80x83                                                                    ⁢                                                                      ω                                    /                                    k                                                                                                                              )                                                                                                      ,                                                                              -                            f1                                                    ⁢                                                      xe2x80x83                                                    ⁢                          sin                          ⁢                                                      xe2x80x83                                                    ⁢                                                      θ                            2                                                                                              }                                                        -                                      j                    ⁢                                          xe2x80x83                                        ⁢                    W                    ⁢                                          {                                              -                                                  f1                          (                                                                                                                    sin                                ⁢                                                                  xe2x80x83                                                                ⁢                                                                  θ                                  1                                                                                            +                                                              n                                ⁢                                                                  xe2x80x83                                                                ⁢                                                                  ω                                  /                                  k                                                                                                                      ,                                                                                          -                                f1                                                            ⁢                                                              xe2x80x83                                                            ⁢                              sin                              ⁢                                                              xe2x80x83                                                            ⁢                                                              θ                                2                                                                                                              }                                                                    ]                                        ⁢                                          exp                      ⁡                                              [                                                                              j                            ⁡                                                          (                                                              n                                -                                m                                                            )                                                                                ⁢                                                      (                                                          f1                              /                              f2                                                        )                                                    ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          α                                                ]                                                                                                                                                    (        9        )            
In the case of ideal imaging ignoring aberration etc., the following equation stands:
xe2x80x83W("xgr",xcex7)=oxe2x80x83xe2x80x83(10)
Therefore, the distribution of intensity I (xcex1, xcex2, xcex81, xcex82) of the individual plane waves can be represented by the following equation:                               I          ⁡                      (                                          θ                1                            ,                              θ                2                                      )                          =                                            ∑              n                        ⁢                                          a                n                2                            ⁢                                                p                  n                                ⁡                                  (                                                            θ                      1                                        ,                                          θ                      2                                                        )                                                              +                      2            ⁢                          ∑                              ∑                                                      a                    n                                    ⁢                                      a                    m                                    ⁢                                                            p                      n                                        ⁡                                          (                                                                        θ                          1                                                ,                                                  θ                          2                                                                    )                                                        ⁢                                                            p                      m                                        ⁡                                          (                                                                        θ                          1                                                ,                                                  θ                          2                                                                    )                                                        ⁢                  cos                  ⁢                                      {                                                                  (                                                  f1                          /                          f2                                                )                                            ⁢                                              (                                                  n                          -                          m                                                )                                            ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      α                                        }                                                                                                          (        11        )            
That is, the above-described equation is weighted in accordance with the ratio of intensities of the respective points of the fly""s-eye lens and superposed to find the overall distribution of the intensity of the light I (xcex1, xcex2) on the wafer surface.
(4) The above-described equation (11) is a basic equation when performing ideal imaging. Below, the above-described equation (11) is actually analyzed. So as to obtain a strict solution, it is necessary to superpose all orders of the deffracted light, but even if attention is paid to only the 0-th order and xc2x11-th order light in actuality, the higher order deffracted light is rejected by the pupil at the problematic fine line widths and therefore generality of the results of analysis is not lost.
(5) When considering only the 0-th order and xc2x11-th order light, due to the positional relationship of the rejection of the deffracted light shown in FIG. 7, only the four following cases exist in equation (12):
(I)
P0=P1=Pxe2x88x921=1
I(xcex81,xcex82)=[a0+2a1 cos{(f1/f2)xcfx89xcex1}]2xe2x80x83xe2x80x83(12)
(II)
P0=P1=1,Pxe2x88x921=0 or P0=Pxe2x88x921=0,P1=0
I(xcex81,xcex82)=a02+a12+2a0a1 cos{(f1/f2)xcfx89xcex1}xe2x80x83xe2x80x83(13)
(III)
P0=1,P1=Pxe2x88x921=0
I(xcex81,xcex82)=a02xe2x80x83xe2x80x83(14)
(IV)
P0=P1=0,Pxe2x88x921=1 or P0=Pxe2x88x921=0,P1=1
I(xcex81,xcex82)=a12xe2x80x83xe2x80x83(15)
The case of (1) described above (equation (12)) is a case where all of the 0-th order and xc2x11-th order lights are fetched and imaging is carried out by three-beam interference. The case of (II) described above (equation (13)) is a case where one of the 0-th order and xc2x11-th order lights is fetched and the imaging is carried out by two-beam interference. The case of (III) described above (the above-described equation (14)) is a case where only the 0-th order light is fetched and occurs under the resolution limit, that is, when the space frequency is high. The case of (IV) described above (the above-described equation (15)) is a case where one of the xc2x11-th order lights is fetched and occurs when the light source is extremely large.
(6) By substituting each of the above-described equations (12) to (15) into the above-described equation (4), the overall distribution of intensity I (xcex1, xcex2) is found. However, the analysis of the above-described equations (12) to (15) suggests how it is possible to use the halftone phase shifting mask method with respect to a design pattern having any pattern density. Namely, when differentiating equation (12), there is a solution with which the cosine term and sine term become 0. This means that where imaging is carried out by three-beam interference, the secondary peak is always generated. Also, even if the above-described equation (13) is differentiated, only a solution with which the sine term becomes 0 exists. This suggests that the secondary peak will not be generated when the imaging is carried out by two-beam interference. Namely, it suggests that, when a light source giving a strong intensity of the light at the center portion of the fly""s-eye lens is used as in the usual stepper, the three-beam interference becomes dominant and a secondary peak is generated, while when a light source giving a strong intensity of the light at the peripheral portion of the fly""s-eye lens is used as in so-called oblique incident illumination, it becomes two-beam interference and a secondary peak is not generated.
(7) With the oblique incident illumination technique, the illumination light is illuminated on the mask surface in an oblique direction, and as a result, either of the 0-th order light or +1-th order light is made incident upon the pupil surface as shown in FIG. 8 and FIG. 9B, the other primary light is rejected by the lens barrel, etc., and the pattern is formed by the two-beam interference. As a result, the apparent numerical aperture of the lens can be reduced and there is a great effect of improvement of the depth of focus. On the other hand, however, there are problems such as an increase of the adjacency effect due to the reduction of the effective light source, a lowering of the illuminance, an increase of the unevenness of illuminance, a lowering of the exposure margin, a strong influence of the telecentricity, etc.
Note that, in FIG. 8, reference numeral 20 denotes a lamp or a laser; reference numeral 21 denotes a collimetor; reference numeral 22 denotes a fly""s-eye lens; reference numeral 23 denotes a condenser lens; and reference numeral 24 denotes a reticle (mask). Also, FIG. 9A shows the case of three-beam interference; and FIG. 9B shows the case of two-beam interference.
(8) When considering the situation of the above-described (6) and (7), it is understood that it is desirable to have a distribution of the intensity of the light on the fly""s-eye lens that emphasizes the oblique incident component to such an extent that a secondary peak, which is a problem in the halftone phase shifting mask, is not generated and has a vertical incident component such that the problem possessed by the oblique incident exposure method is not generated.
(9) Based on the above-described concept, the inventor discovered that the format shown in FIGS. 10A and 10B is suitable as the distribution of the amount of light on the fly""s-eye surface considering, in patterns of all directions, the depth of focus of the periodic pattern, the depth of focus of the isolated pattern, the generation of the secondary peak, the degree of the adjacency effect, the illuminance, the unevenness of illuminance, the distortion change, etc., and thereby completed the present invention. FIG. 10A shows the distribution of the amount of light of the light incident upon the fly""s-eye lens by numerals. The asterisk parts indicate the peak parts of the amount of light and the numerals 0 to 9 show the proportions of the amount of light where the peak part is defined as 10. A distribution of the amount of light is exhibited which is low in the center portion and high in the peripheral portions. Also, FIG. 10B shows three-dimensionally the distribution of the amount of light shown in FIG. 10A.
(10) The present invention can be preferably used in a case where the mask pattern of the semiconductor device is transferred to a resist by using photolithography and in other cases.
Concretely, the method of pattern formation according to the present invention comprises irradiating light from an effective light source to a mask, transferring the pattern of the mask onto a substrate, and forming the pattern on the substrate, wherein the amount of light emitted from a center portion of the aforesaid effective light source is lowered by a certain amount with respect to the amount of light emitted from the peripheral portions of the effective light source.
The amount of the light emitted from the center portion of the effective light source is preferably lowered by 2 to 90 percent with respect to the peak value of the amount of light emitted from the peripheral portions of the effective light source.
As the mask, use may be made of an ordinary chromium mask or phase shifting mask etc. The present invention is particularly effective in the case of use of a phase shifting mask. The phase shifting mask is not particularly limited and may for example be a rim type or outrigger type phase shifting mask.
The effective light source is not particularly limited, but for example suitable use may be made of a fly""s eye lens comprising an assembly of a plurality of lenses. Alternatively, use may be made of a plurality of optical fibers as the effective light source.
The region of the center portion of the aforesaid effective light source where the amount of light is lowered is for example a region of 10 to 40 percent of the outer diameter of the effective light source.
The peak of the amount of light at the peripheral portions of the aforesaid effective light source appears for example at a few points at rotary symmetrical positions at the peripheral portions of the effective light source.
The amount of light incident upon the center portion of the aforesaid effective light source can be lowered in relation to the peak value of the amount of light incident upon the peripheral portions of the effective light source by using a beam splitter and a pyramid or conical prism lens. Further, the amount of the light incident upon the center portion of the effective light source can also be lowered with respect to the peak value of the amount of light incident at the peripheral portions of the effective light source by using a beam splitter alone, or pyramid or conical prism alone, or a plurality of pyramid or conical prisms.
It is also possible to lower the amount of light incident upon the center portion of the aforesaid effective light source lens relative to the peak value of the amount of light incident upon the peripheral portions of the effective light source by using a filter having a high transmittance at the peripheral portions as against the center portion.
It is also possible to lower the amount of light emitted from the center portion of the aforesaid fly""s eye lens relative to the peak value of the amount of light emitted from the peripheral portions of the fly""s eye lens by using a mechanical filter having opening portions corresponding to the individual lenses of the fly""s-eye lens and having diameters of the related opening portions larger at the peripheral portions relative to the center portion.
It is also possible to lower the amount of light incident upon the center portion of the aforesaid effective light source relative to the peak value of the amount of light incident upon the peripheral portions of the effective light source by using a prism and optical parts arranged so that they can be made to approach or move away from this prism in the light path axis direction.
It is also possible to lower the amount of light emitted from the center portion of the aforesaid effective light source relative to the peak value of the amount of light emitted from the peripheral portions of the effective light source by dividing the beam incident upon the aforesaid effective light source to two or more beams, irradiating the two or more divided beams onto the aforesaid effective light source by using a movable mirror, and utilizing the difference in scanning speed at respective points of the surface of the effective light source dependent upon the surface shape of the aforesaid movable mirror.
The diameters of the individual lens of the fly""s eye lens may be uniform or may be larger at the center portion than the peripheral portions. For example, in a fly""s eye lens, the diameters of the individual lens at the center portion may be made 1.1 to 3 times the diameters of the individual lenses at the peripheral portions. By configuring the lens in this way, it is possible to prevent unevenness of illumination.
Further, in the present invention, when a single exposure time is T, it is possible to pass the light through a first filter with a higher transmittance at the peripheral portions than the center portion for a time of 0.05xc3x97T to 0.95xc3x97T and to pass it through a second filter or not pass it through a filter in the remaining exposure time. By this, it is possible to lower the amount of light from the center portion of the effective light source relative to the peak value of the amount of light emitted from the peripheral portions of the effective light source in just a single exposure time.
The apparatus for pattern formation according to the present invention is one irradiating light from an effective light source to a mask, transferring the pattern of the mask onto a substrate, and forming the pattern on the substrate and is provided with a light intensity distribution correcting means for lowering the amount of light emitted from a center portion of the aforesaid effective light source by a certain amount with respect to the amount of light emitted from the peripheral portions of the effective light source.
The light intensity distribution correcting means preferably is a means for lowering the amount of the light emitted from the center portion of the effective light source by 2 to 90 percent with respect to the peak value of the amount of light emitted from the peripheral portions of the effective light source.
The mask is preferably a phase shifting mask.
The effective light source is preferably a fly""s eye lens comprising an assembly of a plurality of lenses. The diameters of the individual lenses of the fly""s eye lens are preferably larger at the center portion than the peripheral portions.
The region of the center portion of the aforesaid effective light source where the amount of light is lowered by the light intensity distribution correcting means is preferably a region of 10 to 40 percent of the outer diameter of the effective light source.
The peak of the amount of light at the peripheral portions of the aforesaid effective light source preferably appears at a few points at rotary symmetrical positions at the peripheral portions of the effective light source.
The light intensity distribution correcting means preferably has an optical element which lowers the amount of light incident upon the center portion of the effective light source in relation to the peak value of the amount of light incident upon the peripheral portions of the effective light source. In a fly""s eye lens, the diameters of the individual lens at the center portion are preferably 1.1 to 3 times the diameters of the individual lenses at the peripheral portions.
The optical element is preferably a beam splitter and/or a prism lens.
The light intensity distribution correcting means preferably has a filter having a high transmittance at the peripheral portions as against the center portion.
The light intensity distribution correcting means preferably has a mechanical filter, which mechanical filter has opening portions corresponding to the individual lenses of the fly""s-eye lens and has diameters of the related opening portions larger at the peripheral portions relative to the center portion.
The light intensity distribution correcting means preferably has an optical element and optical parts arranged so that they can be made to approach or move away from this optical element in the light path axis direction. In this case, a prism, for example, may be used as the optical element.
The light intensity distribution correcting means preferably further has means for dividing the beam incident upon the aforesaid effective light source to two or more beams, a movable mirror for irradiating the two or more divided beams onto the aforesaid effective light source, and scan means for lowering the amount of light emitted from the center portion of the aforesaid effective light source relative to the peak value of the amount of light emitted from the peripheral portions of the effective light source by using the difference in scanning speed at respective points of the surface of the effective light source dependent upon the surface shape of the aforesaid movable mirror.
The light intensity distribution correcting means may also have a first filter with a higher transmittance at the peripheral portions than the center portion and a switching means for switching between a state passing light through the first filter for a single exposure time and a state performing exposure without passing through the first filter. In this case, the apparatus of the present invention preferably further has a second filter with a distribution of transmittance different from the first filter and causes the light to pass through the second filter in the state performing exposure without passing through the first filter.
The switching means preferably has a rotary disk on which at least the first filter is mounted and a drive means for driving the rotation of the rotary disk. Further, the switching means may have a slide mechanism enabling at least the first filter to move slidably. Further, as the switching means, use may be made of an optical shutter type optical material whose transmittance of light changes according to the voltage applied.
The method of production of a semiconductor device according to the present invention is particularly suitable for use when making a semiconductor device on a semiconductor substrate using the above mentioned method of pattern formation.